MDPE Copolymer Composition for Stable High-Throughput Extrusion
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Solution Overview
Problem
Existing polyethylene polymers face challenges in achieving a balance of desirable properties such as density, melt index, and processability, particularly in extrusion processes, leading to issues like melt fracture and poor bubble stability.
Innovation Solution
Development of medium density polyethylene (MDPE) copolymers with a unique balance of long chain branching, higher melt index, and improved processability, characterized by specific density, melt index, and rheological properties, using a metallocene catalyst.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high melt index polymers are used to achieve low melt viscosity in extrusion shear rate regime, then throughput rate is improved, but bubble stability deteriorates due to low zero shear viscosity
Solution Approach 1:
The patent changes the molecular weight distribution parameters by using a dual catalyst system that produces a bimodal MWD. This creates a population of high molecular weight chains that provide high zero shear viscosity for bubble stability, while simultaneously maintaining low molecular weight chains that provide low melt viscosity for high throughput extrusion processing.
2Ease of manufacture
If LLDPE is used instead of LDPE, then molecular weight distribution becomes narrower and processing is improved, but long chain branching is reduced leading to poor bubble stability
Solution Approach 1:
The patent creates a composite molecular structure within the polymer by combining two distinct molecular weight populations through a dual catalyst system. The high molecular weight fraction provides long chain branching characteristics for bubble stability, while the low molecular weight fraction provides processing ease, effectively creating a composite material at the molecular level.
3Strength
If metallocene catalyst is used to produce mLLDPE, then physical properties are improved, but melt strength is reduced and melt fracture occurs at commercial shear rates
Solution Approach 1:
The patent applies local quality by creating different molecular weight regions within the polymer structure. The high molecular weight regions provide melt strength and resistance to melt fracture, while the low molecular weight regions maintain the superior physical properties achieved through metallocene catalysis. This spatial differentiation of molecular weights resolves the conflict between strength and harmful melt fracture effects.
4Reliability
If LDPE is blended with mLLDPE to increase melt strength, then bubble stability is improved, but mechanical properties deteriorate
Solution Approach 1:
The patent extracts the beneficial long chain branching characteristics from LDPE and incorporates them directly into the mLLDPE molecular structure through the dual catalyst system. This eliminates the need for blending with LDPE while maintaining bubble stability, thereby preserving the superior mechanical properties of neat mLLDPE.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The MDPE copolymers exhibit improved extrudability, reduced melt pressures, and enhanced bubble stability during fabrication, allowing for higher throughput rates without sacrificing mechanical properties.
Implementation Method 1
using a metallocene catalyst
Data Source
AI summary
The present disclosure generally relates to polyethylene polymers and to articles made therefrom. In some embodiments, a polyethylene copolymer includes ethylene units, and 1 wt % to 8 wt % of C3-C8 alpha-olefin comonomer units. The polyethylene copolymer has a density of 0.934 g/cm3 to 0.945 g/cm3, a melt index (MI, determined per ASTM D1238 at 190° C. and 2.16 kg loading) greater than 1 g/10 min and less than or equal to 2.5 g/10 min, a composition distribution breadth index of 75% or greater, a molecular weight distribution (MWD, Mw/Mn) of 2 to 8, and a melt index ratio (MIR) within the range from 30 to 75, wherein MIR is the ratio of high load melt index (HLMI, ASTM DI 238 at 190° C., 21.6 kg) to melt index (MI, ASTM DI 238 at 190° C., 2.16 kg).